Psychiatric diseases are chronic, devastating disorders thought to arise from maladaptive brain plasticity, and potent and safe pharmacotherapies are in great need. Identifying the mechanistic links that might sustain these aberrant neuroadaptations will advance our understanding of the biology of mental disorders, potentially providing new platforms for medication development. Using an innovative bioluminescence-based molecular screening approach combined with biochemical, electrophysiological, and imaging assays, we provide breakthrough results showing a link between glycogen synthase kinase 3 (GSK3), a critical enzyme found dysfunctional in mood disorders, depression and schizophrenia, and neuronal excitability, which we propose as a potential mechanism underlying dysfunction of neuronal circuitries associated with psychiatric disorders and certain addictive behaviors. Building on previous discoveries demonstrating that fibroblast growth factor 14 (FGF14) is a functionally relevant component of the Nav channelosome that controls neuronal excitability, we present exciting new data showing that the FGF14:Nav channel complex formation is bi-directionally controlled by GSK3 and by the GSK3 constitutive repressor, protein kinase B (Akt), and that GSK3 directly phosphorylates FGF14. Pharmacological inhibition of Akt and GSK increases and prevents, respectively, the FGF14:Nav channel complex formation, whereas inhibition of GSK3 occludes the effect of Akt inhibition. In hippocampal neurons, GSK3 inhibition disperses the FGF14:Nav channel complex from the axonal initial segment (AIS), the site of action potential initiation, impairs intrinsic fring and reduces excitatory synaptic transmission, whereas inhibition of Akt leads to opposite phenotypes. Furthermore, we show that Fpep1, a small interfering peptide modeled upon the FGF14:Nav channel interface, prevents the FGF14:Nav channel complex assembly, providing a tool for minimizing the effect of GSK3 on neuronal excitability in vivo. In this proposal we will employ a combination of bioluminescence-based technology, mass spectrometry, phosphorylation assays, confocal imaging and electrophysiology to determine the molecular mechanism by which GSK3 controls the FGF14:Nav channel complex formation (Aim 1) and promotes targeting of the FGF14:Nav channel complex in neurons (Aim 2) and to evaluate whether GSK3 exerts an effect on excitability and neuroplasticity in cortico-limbic circuits through the FGF14:Nav channel complex that could be reversed by pharmacological or genetic approaches targeting FGF14 (Aim 3). Positive outcomes of this study will provide new insights into the molecular mechanisms of GSK3 in the brain and offer an unprecedented opportunity for new medication development against GSK3-linked psychiatric disorders.